When you use the WAV file as the Source Data file for the switch layer, it only uses it that one time to create the switch keyframes.
It doesn't load it as the soundtrack.

To hear the sound, you must also load the WAV as the soundtrack. Use the Animation menu, "Select Soundtrack ..." option. Then play it back.

Moho currently only handles sound playback by loading a single soundtrack file. Mixing sounds requires editing with a separate audio or audio/video editor, although Brian (7feet) Quinn is experimenting with a mixing script in the scripting section of the forum.